The present invention relates to a tension control apparatus for vibrating screens. More particularly, the present invention relates to an improved tension control apparatus which provides a substantially uniform tension across the width of a vibrating screen.
Vibrating screen assemblies for separating particulate matter into various sizes are well known. Conventional vibrating screen assemblies include a generally rigid frame, a screen deck coupled to the frame, and a vibrating mechanism for vibrating the screen deck.
It is often difficult to adjust the tension of screens that form the screen deck. In order to facilitate adjustment of the tension of screens in conventional vibrating screen assemblies, various types of tension control mechanisms have been developed. For example, a first end screen is coupled to the frame and a second end of the screen is coupled to a tension plate. Stationary coil tension springs are coupled to the tension plate to draw the screen over a series of vibrating support bars arranged in an arc. As the screen becomes stretched, the coil tension springs automatically keep the tension in the screen substantially constant. This reduces whipping or flexing of the screen which can cause wire breakage. See, for example, U.S. Pat. Nos. 4,137,157 and 2,804,208.
It is also known to provide a rotatable tension adjustment apparatus rotatably coupled to the frame. The rotatable tension adjustment apparatus is typically coupled to one end of the screen to facilitate adjusting the tension of the screen. Tension of the screen is adjusted as the tension adjustment apparatus is rotated relative to the frame. See, for example, U.S. Pat. Nos. 2,000,426; 2,338,523; 4,529,510; 4,732,670; and 4,906,352.
The tension adjustment apparatus disclosed in U.S. Pat. No. 4,906,352 includes an anchor assembly coupled to one end of a screen and a rotatable tension assembly spaced apart from the anchor assembly coupled to another end of the screen. The rotatable tension assembly includes an elongated tube rotatably coupled between spaced apart side walls of a rigid frame. As the tube is rotated to adjust the tension of the screen, the tube can bend in the middle. Therefore, the tension of the screen adjacent the bent middle portion of tube is less than the tension of the screen near the rigid side walls of the frame. To compensate for the reduced tension in the middle portion of the screen, the apparatus disclosed in the '352 Patent includes a second tension adjustment mechanism coupled to the anchor assembly. The second tension adjustment mechanism includes a flexible channel attached to opposite side walls of the frame and a rigid beam which is also attached to opposite side walls of the frame. The channel engages a screen flange to couple the first end of the screen to the channel. The second tension adjustment mechanism also includes a plurality of brackets coupled to the rigid beam and a bolt coupled between each bracket and the channel. By tightening the selected bolts, an operator can bend a predetermined portion of the flexible channel of the anchor assembly to increase the tension on the screen adjacent the predetermined portion of the channel. In other words, an operator can bend the channel to match the bend in the rotating tube at the opposite end of the screen to compensate for the reduced tension in the bent portion of the tube. The second tension adjustment mechanism disclosed in the '352 patent is located near the middle of the frame spaced apart from an end edge. Therefore, access to the second adjustment mechanism disclosed in the '352 patent can be difficult.
The present invention is designed to provide an improved tension control apparatus for a vibrating screen. The present invention includes a rotatable tension plate for adjusting the tension of a screen and means for selectively and incrementally adjusting the relative rigidity of the tension plate at predetermined intervals along the width dimension of the screen to compensate for nonuniform tension of the screen. The adjusting means is provided to straighten the tension plate if the tension plate bends during tensioning of the screen to provide a substantially uniform tension across the width of the screen. Advantageously, by providing an adjusting apparatus coupled to the rotatable tension plate at the same end of the screen, the adjusting apparatus is easily accessible in vibrating the screen assemblies having one or two screens. In addition, instead of bending two beams to match each other, the present invention straightens the rotatable tension plate. Therefore, both ends of the screen are aligned in substantially straight lines. This reduces forces acting on the screen which may tend to weaken the screen.
According to the present invention, a vibrating screen apparatus includes a frame and a screen having a first end, a second end, and a width dimension, and an anchor beam rigidly coupled to the frame. The first end of the screen is coupled to the anchor beam. The apparatus also includes means for tensioning the screen to substantially a predetermined tension. The tensioning means is rotatably coupled to the frame spaced apart from the anchor beam. The second end of the screen is coupled to the tensioning means. The tensioning means changes the tension of the screen upon rotation of the tensioning means relative to the frame. The apparatus further includes means for selectively and incrementally adjusting the relative rigidity of the tensioning means at predetermined intervals along the width dimension of the screen to compensate for nonuniform tension of the screen. The adjusting means is coupled to the tensioning means.
According to one aspect of the present invention, the frame includes first and second side walls which are spaced apart by a predetermined distance. The tensioning means includes an elongated pipe rotatably coupled between the first and second side walls of the frame about an axis of rotation. A tension plate is coupled to the pipe and to the second end of the screen. The apparatus includes means for rotating the pipe and tension plate about the axis of rotation to move the second end of the screen relative to the frame to adjust the tension of the screen. The means for rotating the pipe and the tension plate illustratively includes a lever coupled to the pipe and means for rotating the lever relative to the frame. The means for rotating the lever is coupled between the lever and the frame.
According to another aspect of the present invention, the adjusting means includes means for moving the tension plate relative to the pipe in a direction normal to the axis of rotation of the tension plate to compensate for nonuniform tension of the screen. The moving means is coupled to the tension plate. The moving means includes a plurality of adjustable fasteners extending through the tension plate and the pipe at spaced apart intervals to permit adjustment of the position of the tension plate relative to the pipe adjacent each of the plurality of adjustable fasteners.
Each of the plurality of adjustable fasteners includes a bolt extending through the tension plate and the pipe at spaced apart intervals, a rubber tension spring coupled to the bolt, and a nut threadably coupled to the bolt to permit adjustment of the position of the tension plate relative to the pipe. Each of the plurality of adjustable fasteners also includes a spherical socket coupled to the bolt adjacent to the rubber tension spring. Each spherical socket includes a generally spherical concave surface and each of the rubber tension springs includes a generally spherical convex surface configured to engage the generally spherical concave surface of an adjacent spherical socket. The spherical socket and the spherical convex surface of the rubber tension spring provide a flush contact surface despite slight movement of the tension plate relative to the pipe. The resilient rubber tension springs maintain a predetermined tension on the screen even if the screen stretches during operation.
According to yet another aspect of the present invention, a face plate is coupled to the pipe opposite the tension plate. The face plate provides a flat surface to support the tension adjusting means. The spherical socket of the adjusting means abuts the face plate to facilitate movement of the tension plate. The face plate is formed to include a plurality of notches to prevent particulate matter from becoming trapped between the tension plate and the face plate.